Tecnai Standard Operating Procedure

What I've learned from the few sessions I've had on the Tecnai at Penn State's Materials Characterization Lab so far is that using a TEM is really, really confusing. And being told again and again that the instrument is very expensive and that you must be very careful only adds to the anxiety!

That being said, I've decided to write up a procedure for alignment on the Tecnai in as much detail as I could. I am not an experienced user by any stretch of the word, and I do suggest that you use what I've provided here in conjunction with your own notes from your training session, but I hope that these steps can help you in some way.

Please note that this procedure only applies to basic imaging on the Tecnai (I know, I know, these steps look anything but "basic.") Other techniques, such as electron diffraction or electron energy-loss spectroscopy, may require additional steps. I'll be sure to update this post when I eventually learn how to do those!

What are block copolymers and how do they self-assemble?

The following post is a modified version of a short paper I wrote for MatSE 542 last semester. It is essentially a summary of the paper “Self-assembly of block copolymers” by Yiyong Mai and Adi Eisenberg published in The Royal Society of Chemistry in 2012. 

Introduction
Block copolymers (BCPs) are a fascinating class of materials that have recently attracted significant attention due to their ability to self-assemble into a variety of morphologies, such as spheres, cylinders, gyroids, and lamellae. This post will discuss what block copolymers are, the thermodynamics behind microphase separation, and their theoretical and experimental phase diagrams.

Simply put, block copolymers consist of two or more chemically dissimilar polymer blocks that are thermodynamically immiscible yet covalently bonded. Figure 1 below illustrates the most popularly studied structures of block copolymers which can be formed with two types of blocks, A and B. Such a material is referred to as a diblock copolymer, while structures consisting of three blocks are called triblock copolymers, and so on. The chemically distinct blocks will separate into different domains while the covalent bonds restrict this demixing to local length scales, resulting in what is called microphase separation and giving rise to the aforementioned myriad of morphologies. 

Figure 1: Schematic of different structures of diblock copolymers. Reproduced with permission from The Royal Society of Chemistry.

Microphase separation

Microphase separation in BCPs is driven by a combination of the unfavorable mixing enthalpy and a small mixing entropy, with the covalent bonds holding blocks together to prevent macroscopic phase separation. For a diblock copolymer consisting of blocks A and B, microphase separation is influenced by three parameters: the volume fractions of the A and B blocks (f_A and f_B), the total degree of polymerization of the two blocks (N = N_A + N_B), and the Flory-Huggins interaction parameter (χ_AB). The Flory-Huggins parameter is dependent upon several factors and can be described with the equation below:

where z is the number of nearest neighbors per repeat unit, k_B is the Boltzmann constant, T is the temperature, and ε is the interaction energy of the respective block pairs. The degree of microphase separation is determined by the product of the interaction parameter χ_AB and the total degree of polymerization N. Thus, as χN decreases (or as temperature increases), the blocks become increasingly miscible while the combinatorial entropy increases and the copolymers become disordered. This behavior is referred to as an order-to-disorder transition (ODT).